Following traumatic or mechanically induced axonal degeneration in the peripheral nervous system, axonal regeneration ensues, resulting in functional recovery. However, the rate of axonal elongation (3-4 mm/day) is slow. Consequently, recovery is measured in weeks or months, depending upon the distance between the site of injury and the target tissue. Therapies that speed regeneration over long distances would be highly beneficial to patients and would significantly reduce health care costs.
The immunosuppressant drug FK506 (USAN tacrolimus; Prograf®) possesses a variety of neuronal properties, including protection against ischemic brain injury (Sharkey and Butcher, Nature 371:336-339, 1994; Ide et al., Neurosci. Lett. 204:157-160, 1996; Tokime et al., Neurosci. Lett. 206:81-84, 1996) and glutamate toxicity in vitro (Dawson et al., Proc. Natl. Acad. Sci. USA 90:9808-9812, 1993), prevention of N-methyl-D-aspartate (NMDA) receptor desensitization (Tong et al., Science 267:1510-1512, 1995), prevention of kindling (Moriwaki et al., Neurosci. Res. 25:191-194, 1996), blockade of long-term potentiation (LTP) and long-term depression (LTD) in the visual cortex (Torii et al., Neuroscience 185:1-4, 1995; Funauchi et al., Neurosci. Res. 19:269-278, 1994), facilitation of LTP (Ikegami et al., Mol. Brain Res. 41:183-191, 1996) and blockade of LTD in the rat hippocampus (Hodgkiss and Kelly, Brain Res. 705:241-246, 1995), and alteration in neurotransmitter release (Steiner et al., Mol. Med. 96:1076-1151, 1996) and endocytosis (Kuromi et al., Neurosci. Res. 27:101-113, 1997). It is likely that these neuronal properties of FK506 are mediated by calcineurin inhibition. FK506 also speeds functional recovery and axonal regeneration in the rat in a dose-dependent manner following a sciatic nerve crush lesion (Gold et al., J. Neurosci. 15,7505-7516, 1995; Gold et al., Restor. Neurol. Neurosci. 6:287-296, 1994). FK506 was shown to stimulate neuritic outgrowth in a rat pheochromocytoma cell line in a concentration-dependent manner (Lyons et al., Proc. Natl. Acad. Sci. USA 91:3191-3195, 1994).
FK506 and cyclosporin A share a common mechanism for producing immunosuppression; both inhibit the protein phosphatase calcineurin following binding to their respective immunophilins, FK506-binding protein-12 (FKBP-12) and cyclophilin A (Harding et al., Nature 341:758-760, 1989; Siekierka et al., Nature 341:755-757, 1989; Sigal and Dumont, Annu. Rev. Immunol. 10:519-560, 1992; Snyder and Sabatini, Nature Medicine 1:32-37, 1995; Wiederrect and Edzkorn, Perspectives in Drug Discovery and Design 2:57-84, 1995). It has been thought that the activity of FK506 in promoting nerve regeneration and growth is also related to the binding of FKBP-12.
There has been an intense effort to design compounds that are structurally related to FK506 and that share FK506's ability to inhibit FKBP-12 and thereby produce immunosuppression. See, for example: Bierer et al., Science 250:556-559, 1990; Van Duyne et al., Science 252:839-842, 1991; Van Duyne et al., J. Mol. Biol. 229:105-124, 1993; Hauske et al., J. Med. Chem. 35:4284-4296, 1992; Holt et al., J. Am. Chem. Soc. 115:9925-9938, 1993; Holt et al., Bioorg. Med. Chem. Lett. 3:1977-1980, 1993; Teague and Stocks, Bioorg. Med. Chem. Lett. 3:1947-1950, 1993; Wang et al., Bioorg. Med. Chem. Lett. 4:1161-1166, 1994; Yamashita et al., Bioorg. Med. Chem. Lett. 4:325-328, 1994; Stocks et al., Bioorg. Med. Chem. Lett. 4:1457-1460, 1994; Goulet et al., Perspect. Drug Disc. Design 2:145-162, 1994; Wilson et al., Acta Cryst. D51:511-521, 1995; Armistead et al., Acta Cryst. D51:522-528, 1995; U.S. Pat. Nos. 5,192,773, 5,330,993, 5,516,797, 5,612,350, 5,614,547, 5,622,970, 5,654,332; and published international patent applications WO 92/00278, WO 92/04370, WO 92/19593, WO 92/21313, WO 94/07858, and WO 96/40633 (Hamilton and Steiner).
Snyder and co-workers have reported (Steiner et al., Nature Medicine 3:1-8, 1997; Steiner et al., Proc. Natl. Acad. Sci. USA 94:2019-2024, 1997) that systemic administration (via subcutaneous injection) of two such molecules that bind FKBP-12 (with binding affinities of 25 and 250 nM, respectively) but that do not inhibit calcineurin activity (and which are not immunosuppressants) increase the size of myelinated fibers.
U.S. Pat. No. 5,654,332 (Armistead et al.) discusses immunosuppressive compounds that bind FKBP-12 and that are said to stimulate neurite outgrowth in the presence of NGF. It was stated that the neurotrophic activity of these FKBP-12 binding compounds “is directly related to their affinity for FKBP-12 and their ability to inhibit FKBP-12 rotomase activity” (id. at col. 7, lines 47-50).